P. Guillaume

Force identification by means of in-operation modal models

In-operation modal analysis has become a valid alternative for structures where a classic forced-vibration test would be difficult if not impossible to conduct. The modelling of output-only data obtained from naturally excited structures is particularly interesting because the test structure remains in its normal in-operation condition during the test. One of the drawbacks of in-operation analysis is that part of the modal parameters can no longer be estimated. Consequently, the applicability of in-operation modal models remains somewhat restricted. For some in-operation applications, interest lies in the identification of forces that gave rise to measured response signals. In order to solve this ill-conditioned problem, a complete modal model of the structure is required. Recently, a sensitivity-based method was proposed for the normalization of operational mode shape estimates on a basis of in-operation modal models only. This method allows the reconstruction of complete modal models from output-only data. In this contribution, the possibility of using such re-completed in-operation modal models for the identification of localized forces is explored.

An automatic frequency domain modal parameter estimation algorithm

In this article, a frequency-domain modal parameter estimation method is proposed. The algorithm automatically separates physical poles from mathematical ones. An important issue in the automatization of the algorithm is the inclusion of noise information to estimate the standard deviations of the poles. These standard deviations are used (together with other features) as the inputs of a fuzzy clustering algorithm. The clustering algorithm then classifies the poles into the mathematical and physical ones. The method requires no user interaction, and a parameter is available quantifying the success of the classification.

Identification of modal parameters including unmeasured forces and transient effects

In this paper, a frequency-domain method to estimate modal parameters from short data records with known input (measured) forces and unknown input forces is presented. The method can be used for an experimental modal analysis, an operational modal analysis (output-only data) and the combination of both. A traditional experimental and operational modal analysis in the frequency domain starts respectively, from frequency response functions and spectral density functions. To estimate these functions accurately sufficient data have to be available. The technique developed in this paper estimates the modal parameters directly from the Fourier spectra of the outputs and the known input. Instead of using Hanning windows on these short data records the transient effects are estimated simultaneously with the modal parameters. The method is illustrated, tested and validated by Monte Carlo simulations and experiments. The presented method to process short data sequences leads to unbiased estimates with a small variance in comparison to the more traditional approaches.

Identification of Young's modulus from broadband modal analysis experiments☆

Abstract The stress–strain relationship of linear viscoelastic materials is characterised by a complex-valued, frequency-dependent elastic modulus E ( jω ) (Youngs modulus). Using system identification techniques it is shown in this paper how E ( jω ) can be measured accurately in a broad frequency band from forced flexural (transverse) and longitudinal vibration experiments on a beam under free–free boundary conditions. The advantages of the proposed method are (i) it takes into account the disturbing noise and the non-linear distortions, (ii) E ( jω ) is delivered with an uncertainty bound, (iii) the low sensitivity to non-idealities of the experimental set-up, and (iv) the ability to measure lowly damped materials. The approach is illustrated on brass, copper, plexiglass and PVC beams.

Modal parameter estimation and monitoring for on-line flight flutter analysis

The clearance of the flight envelope of a new airplane by means of flight flutter testing is time consuming and expensive. Most common approach is to track the modal damping ratios during a number of flight conditions, and hence the accuracy of the damping estimates plays a crucial role. However, aircraft manufacturers desire to decrease the flight flutter testing time for practical, safety and economical reasons by evolving from discrete flight test points to a more continuous flight test pattern. Therefore, this paper presents an approach that provides modal parameter estimation and monitoring for an aircraft with a slowly time-varying structural behaviour that will be observed during a faster and more continuous exploration of the flight envelope. The proposed identification approach estimates the modal parameters directly from input/output Fourier data. This avoids the need for an averaging-based pre-processing of the data, which becomes inapplicable in the case that only short data records are measured. Instead of using a Hanning window to reduce effects of leakage, these transient effects are modelled simultaneously with the dynamical behaviour of the airplane. The method is validated for the monitoring of the system poles during flight flutter testing.

Increased reliability of reference-based damage identification techniques by using output-only data

Abstract During the past few years, a considerable number of damage identification techniques have been proposed and successfully tested on vibration data obtained from mechanical structures. Most vibration-based methods identify damage by interpreting measured changes in modal parameters. In practice, damage identification problems can occur due to minor changes in the boundary conditions of the test set-up especially if classic input–output vibration measurements are required for the diagnosis of a lightweight structure. In this contribution, a comparison is made between an input–output and output-only damage identification set-up for an aluminum beam structure suffering from fatigue-induced crack formation.

User-assisting tools for a fast frequency-domain modal parameter estimation method

Abstract Recently, the least-squares complex frequency-domain (LSCF) estimator has been developed for modal analysis applications. This contribution elaborates in more detail the fast derivation of stabilisation charts and uncertainty bounds for the estimated poles. An alternative representation for the stabilisation chart as well as a robust cluster algorithm to identify clusters of poles using the chart information is presented. Based on the clusters, uncertainty bounds for the poles and an automation of the pole selection process are derived. The relation of these “variances” with the stochastic variances (or confidence bounds) introduced by the noise on the measurements is compared by means of Monte-Carlo simulations. The use of alternative representation for the stabilisation chart in combination with the robust cluster analysis as well as the availability of uncertainty bounds for the modal parameters, assist the user with the performance of an accurate modal parameter estimation.

Combined damage detection techniques

Abstract Linear damage detection techniques are used frequently because of their simplicity and their easy interpretation. In this paper, it will be shown however that linear techniques are not very robust with respect to environmental changes and interstructure variability. With the aid of experimental results it will be demonstrated that non-linear damage detection techniques, although being more complex, are less sensitive to these effects. In addition, two damage detection approaches will be proposed that combine the advantages of different classes of techniques. Firstly, a combined linear–non-linear approach is described. In the second proposed method, static and dynamic measurement techniques will be combined. Using experimental damage detection results, it will be shown that both proposed combined techniques are less sensitive to environmental changes while leading to easy interpretation of results.

The use of multisine excitations to characterise damage in structures

In order to detect the presence of damage and imperfections in materials, a new and promising method for non-destructive material testing has been developed. The technique focuses on the non-linear distortions that are present in the results of a frequency response function (FRF) or transfer function measurement of the sample. The kernel idea in the described method is to use well-chosen periodic excitations where only some of the considered frequency components are excited. The non-excited frequency lines are used to detect, qualify (even or odd non-linear distortions) and quantify (What is the level of the non-linear distortions?) the non-linear distortions. Undamaged materials are often essentially linear in their response. However, the non-linear behaviour of the same material increases significantly when damage appears. The method is applied in the field of damage detection and health monitoring. The method is illustrated by experiments on uncracked and cracked artificial slate beams used in civil constructions and during mechanical cyclic fatigue loading. The developed technique demonstrated to be a very fast and efficient tool to assess global damage in a material.

Continuous-Time Operational Modal Analysis in the Presence of Harmonic Disturbances

Operational modal analysis (OMA) allows to identify the modal parameters from the measured response to unknown random perturbations of a mechanical structure in operation. However, in all applications with rotating components (e.g. helicopters, turbines, diesel motors, ...), the structural vibration in operation is a combination of the response to the random perturbation and the harmonic excitation due to the rotating components. Classical OMA methods fail if the harmonic disturbance is close to, or coincides with a resonance frequency of the structure. Therefore, these methods have been extended to deal with harmonic disturbances with a known, fixed frequency. However, in many applications (e.g. helicopters, wind turbines, diesel motors, ...) the frequencies of the harmonic disturbances vary in time. This paper presents three methods for suppressing the influence of harmonic disturbances with unknown varying frequencies in operational modal analysis. Two of these methods can handle the case where the peak of the harmonic disturbance and the resonance peak completely overlap. The performance of the three methods is illustrated on simulations and real helicopter data.